1. Field of the Invention
The invention relates to a method for change of section of a billet of a continuous casting device operating by continuous casting, wherein opposite sides of the billet below the casting die interact with oppositely positioned roll supports of a billet guide which are divided into a sequence of segments that support rolls. Each segment can be separately adjusted with regard to its angle relative to the billet. In an initial position the entire strand guide to be adjusted for section change is adjusted to a production format thickness.
2. Description of the Related Art
In a method disclosed in EP 0 286 862 A1 the billet, which is cast in a continuous casting die and has a thickness of 40 to 50 mm, is compressed after exiting the casting die by a pair of rolls such that the inner walls of the billet shell formed in the casting die are welded together.
During continuous casting in a continuous casting die of a certain length the thickness of the formed billet shell depends substantially on the casting speed. For ensuring a constant roll gap of the roll pair, the rolling force must be adjusted to the current thickness of the billet shell. When the casting speed is too minimal, the applied rolling force is not sufficient so that the nominal thickness of the produced billet is surpassed. When the casting speed is too high, welding of the billet shell can be achieved only by falling below the nominal thickness of the produced casting billet.
In order to prevent an undesired thickness deviation of the produced cast billet and to achieve a good inner microstructure, in a method disclosed in EP 0 535 368 B1 the thickness of the cast billet is reduced by roll forming and the billet is subsequently rolled, wherein the cast billet is comprised of a solidified billet shell and a liquid core. In this connection, the cast billet is cast with a thickness of 40 to 80 mm, and, subsequently, the billet is roll-formed by maximally three passes to a thickness of 15 to 40 mm and to 2 to 15 mm residual liquid core. Subsequently, the billet is divided into conventional billet lengths, is heat-treated in a compensation furnace, and, subsequently, for example, hot-rolled by reversing rolling. In this method for thickness reduction of the cast billet below the casting die, a transition piece with a reduced thickness results which cannot be rolled but must be separated from the billet and then cut as scrap metal. The reduced thickness moreover results in the transition piece being relatively cold so that the scissors during cutting are greatly loaded.
On the other hand, the changes of section in the continuous casting device during the casting operation are a necessary requirement for production optimization. Accordingly, the patent literature discloses various methods.
The patent DE 43 38 815.2 describes a method and a device for operating a continuous casting device, in particular, during cast-on of the continuous casting device for producing thin billets for hot roll treatment, with at least one reduction roll pair downstream of the continuous casting die followed by adjustable billet guiding elements. The reduction roll pair is adjusted, after a predetermined pass length of the cast billets, to a smaller gap width which results in squeezing off the sump. The cast billet is deformed to a cast-on format having a thickness below the thickness of the desired final format. The billet guiding elements or the reduction roll pair is then adjusted to the thickness of the final format as soon as the less thick cast-on format has reached completely its adjusting range. The reduction roll pair is pressure-controlled and after adjustment of the billet guide is positioned to match the final format.
EP 0 743 116 A1 discloses a vertical casting line for cast billets, comprising a casting die as well as a component group with foot rolls adjoining the outlet of the casting die, and, moreover, a plurality of guide units, correlated roll segments as well as a driver arrangement in connection with a horizontal segment of the casting line. The guide elements comprise at least the entire vertical segment of the casting line wherein at least some of the rolls of the guide units cooperate with adjusting devices which are governed by a process data unit in order to ensure a controllable soft reduction at least within the second part of the vertical segment.
The published document DE 196 39 297 A1 describes a method and a device for high-speed continuous casting devices with a billet thickness reduction during solidification. In the method and the corresponding device for continuous casting of billets, the billet cross-section is thickness-reduced linearly during solidification across a minimum length of the billet guide directly underneath the casting die. With the subsequent further billet cross-section reduction across the residual billet guide, the so-called soft reduction, up to maximally directly before the final solidification or the sump tip, a critical deformation of the billet, taking into account the casting speed as well as the steel quality, can be eliminated.
The patent document EP 0 450 391 B1 discloses a device for supporting a cast metal billet, in particular, for soft reduction, in a steel strip casting device, wherein below the billet casting die at both sides of the cast billet mirror-symmetrically positioned roll supports are provided whose rolls interact with the cast billet. Each roll support is arranged on a fixed frame and divided into several roll-supporting segments which are connected to adjusting devices. The segments are coupled to one another in a jointed fashion such that each segment can be individually adjusted at any desired angle relative to the cast billet and that the upper adjusting device provides for a general adjustment of the roll support. This can be a mechanical, hydraulic, or mechanical-hydraulic adjusting device.
It is an object of the present invention to provide a method as well as a device for performing the method suitable for format thickness changes of the cast billet of a continuous casting device operating in continuous casting operation, wherein the casting speed for the transition process for a format thickness (section) change must not be reduced, i.e., constant production and casting conditions should be maintained. The transition length of the billet during the format thickness change should be comparatively shortened for avoiding production losses. In all transition situations of the format thickness change for reducing the break-out risk, an optimal billet support should be ensured. Cold reduced thickness portions, which highly mechanically load the cutting shears, are to be avoided.
In accordance with the present invention, this is achieved in that the change of section (change of the billet format thickness) is carried out in a controlled sequence of adjusting steps of the segments n=1 to n=i, wherein for a section reduction a sequential adjustment in the casting direction of the successively arranged segments (n=1 to i) toward the billet and for a section increase a sequential adjustment in the casting direction of the successively arranged segments (n=1 to i) away from the billet is carried out and that a transition length of the thickness change (section change) is produced which is at least 50%, preferably 80 to 90%, of the billet length.
With the measures according to the invention for performing a section change of the cast billet, it is achieved that the casting speed, for example, for a change from a thicker format to a thinner format or from a thinner format to a thicker format, must not be changed and the casting conditions can be maintained substantially constant. Crop losses, which mean production losses, are avoided. Cold, reduced thicknesses at the leading end or the trailing end of the billet, which cause an unnecessary mechanical loading of the cutting shears, are prevented. The thickness or section changes can be carried out continuously within a wide adjusting range as a function of the respective production program and thus result in a high flexibility of the device. A special advantage is the adaptation of the cast-on thickness in the rolling mill to the required final rolling thickness.
In one embodiment of the invention it is suggested that the cast billet is adjusted to a constant thickness at the leading end of the billet over a length of approximately 1 to 4 meters, and, moreover, it is suggested that the trailing end of the cast billet is adjusted to a constant thickness over a length of approximately 0.5 m to 2 m.
As a further development of the invention, it is suggested that the length of the transition section of the thickness or section change follows in the longitudinal direction a defined wedge shape with a defined billet profile. This means that between the billet beginning and the billet end, each having a constant thickness, a transition format having a wedge shape is arranged so that the resulting billet wedge can be rolled while maintaining an acceptable billet profile. The transition length depends on the amount of thickness reduction and the adjusting parameters. With this method, even in the case of great thickness changes of the billet of approximately 25 mm, the thickness gradient is sufficiently small so that no thickness tolerance problems occur during rolling.
In an essential further development of the principle of the invention, for a billet guide which is comprised of, for example, four segments n=1 to n=4, the defined wedge of the billet is produced with a defined billet profile with the following adjusting steps.
Starting at a certain length, the segment 1 at the exit side and the segment 2 at the entry side are advanced toward or moved away from the billet. Subsequently, with time delay or with a travel delay, i.e., delay for traveling the length of the segment 2, the segment 2 at the exit side and the segment 3 at the entry side are advanced toward or moved away from the billet. Subsequently, again delayed by the travel length of the segment 3, the segment 3 at the exit side and the segment 4 at the entry side are advanced toward or moved away from the billet. Subsequently, the exit side of segment 4, again delayed by the travel length of the segment 4, is advanced toward or moved away from the billet. Advantageously, all segments are moved until they have been adjusted to the final (target) thickness. The adjusting steps are performed simultaneously or substantially simultaneously. The adjusting speed is comparatively minimal and is, for example, less than 2.5 mm/min. With such an operation, a billet profile is produced which has no unacceptable thickness differences center/edge for rolling.
One embodiment of the method proposes that for a section change increase (thickness increase) at constant casting speed, wherein the sump tip of the billet, for example, is within the segment n=3, in a first adjusting phase the first segment n=1 at the exit side is moved away from the billet by means of the joint connection together with the segment n=2 at the entry side by set-point control and, after reaching the target position, i.e., the segment position for the target or final format, in a second adjusting phase the exit side of segment n=2 is moved away from the billet with the entry side of segment n=3 and that successively, in a sequence of identical steps, the adjustment of the segments n=3, 4 to i to the target position is carried out.
According to a further embodiment of the method according to the invention, the advancement of the segments toward the billet is carried out at a constant speed by means of dynamic control, wherein a force threshold to be determined is not to be surpassed.
Furthermore, it is suggested according to the invention that the advancing speed of the segments is calculated with consideration of the permissible billet elongation limits and the actual casting speed in connection with the actual format adjustment or based on the resulting volume stream of the billet.
Advantageously, the adjusting speed is calculated based on the actual casting speed, the segment length, and the required adjusting travel according to the following formula:
V=Ds/Ls*Vcast
wherein Ds is the format thickness change (change of section), Ls is the segment length, and Vcast is the current casting speed.
Further embodiments of the method propose that the adjusting process is monitored, for example, by means of actual cylinder pressures of hydraulic adjusting devices and, upon surpassing a threshold, position control is switched to force control and that, after reaching the target position, the system is switched back to position control.
Details, features, and advantages of the invention result from the following discussion of an embodiment schematically illustrated in the drawings.